Studies in our laboratory have focused on defining the nature of the B cell and T cell defects in Common Variable Immunodeficiency (CVI), a heterogeneous group of primary acquired human immunodeficiency states characterized by hypogammaglobulinemia and impaired functional antibody responses, at the cellular level, and identifying molecular mechanisms responsible for these defects. Studies of purified B cells of patients with CVI show that although the cells have a normal capacity to proliferate, they manifest differentiation defects at multiple levels. Compared with controls, circulating CVI B cell populations contain reduced numbers of sIgG+ and sIgA+ cells with a commensurate increase in sIgM+ B cells, suggesting an in vivo defect in isotype switch. In addition, CVI B cells manifest Ig secretion defects on stimulation. These Ig secretion defects are not overcome by addition of a variety of cytokines. In further studies we show that despite the above abnormalities, CVI B cells are induced to express normal or near-normal levels of C mu, C gamma, and C alpha mRNA after 7 days of stimulation with anti-CD40 and IL-10. That this C-H mRNA expression represents a recovery of CVI B cell differentiation is supported by studies of Ig secretion in which CVI B cells that are first stimulated for 7 days with anti-CD40 and IL-10 and then restimulated in coculture with activated normal allogeneic T cells and IL-10, secrete substantial levels of IgM and IgG and increased amounts of IgA. Overall, therefore, CVI B cell function can be significantly improved by maintenance in culture. These data suggest the abnormalities of B cell differentiation in CVI are reversible and that the defect is a form of B cell anergy. Commensurate with this hypothesis, we have done further experiments whose preliminary results, in fact, indicate that there is a defect in the ability of CVI B cells to act as antigen presenting cells. A second project focuses on X-linked agammaglobulinemia (XLA), another major humoral immunodeficiency syndrome. In affected individuals, B cell development is arrested at the pre-B cell stage with absence of mature B cells, resulting from mutations in a gene termed btk whose product normally exhibits tyrosine kinase. In collaboration with others, we have begun working toward gene therapy of XLA. presently, we have developed a retroviral construct with the btk gene and confirmed its successful transfection into NIH 3T3 fibroblast cells by polymerase chain reaction (PCR).